The present invention relates to an image display and a method of driving the same, and particularly to a technology effective for application to a display apparatus which has thin-film electron emitters having an electrode-insulator-electrode structure to emit electrons into vacuum.
The thin-film electron emitters are electron-emitter elements each using hot electrons produced by applying a high electric field to an insulator.
As a typical example, an MIM (Metal-Insulator-Metal) electron emitter comprising a three-layer thin-film structure of a top electrode-insulating layer-base electrode will be explained.
FIG. 13 is a diagram for describing the principle of operation of an MIM electron emitter illustrated as a typical example of a thin-film electron emitter.
A driving voltage is applied between a top electrode 11 and a base electrode 13 to set an electric field in a tunneling insulator 12 to 1 MV/cm to 10 MV/cm and over. Thus, electrons placed in the neighborhood of a Fermi level in the base electrode 13 are transmitted through a barrier by tunneling phenomena. Thereafter they are injected into a conduction band of the tunneling insulator 12 and further injected into the top electrode 11, thus resulting in hot electrons.
Some of these hot electrons are subjected to scattering under interaction with a solid in the tunneling insulator 12 and the top electrode 11, thus leading to the loss of energy.
As a result, hot electrons having various energies exist when they have reached an interface between the top electrode 11 and vacuum 10.
Of these hot electrons, ones having energy of a work function φ or more of the top electrode 11 are emitted into the vacuum 10, and ones other than the above ones flow into the top electrode 11.
Assuming that a current based on the electrons that flows from the base electrode 13 to the top electrode 11, is called a diode current (Id), and a current based on the electrons emitted into the vacuum 10 is called an emission current (Ie), an electron emission efficiency (Ie/Id) ranges from about 1/103 to about 1/105.
Incidentally, the MIM thin-film electron emitter has been described in, for example, Japanese Patent Application Laid-Open No. Hei 9-320456.
Now, the top electrode 11 and the base electrode 13 are provided in plural form and these plural top electrodes 11 and base electrodes 13 are made orthogonal to one another to thereby form thin-film electron emitters in matrix form. Consequently, electron beams can be produced from arbitrary locations and hence they can be used as electron emitters for a display apparatus.
Namely, a display apparatus can be constructed wherein thin-film electron-emitter elements are placed every pixels and electrons emitted therefrom are accelerated in vacuum and thereafter applied to each of phosphors to thereby allow the applied phosphor to emit light, whereby a desired image is displayed thereon.
The thin-film electron emitters have excellent features as electron-emitter elements for the display apparatus in that they are capable of implementing a high-resolution display apparatus because the emitted electron beams are excellent in directionality, and they are easy to handle because they are insusceptible to the influence of their surface contamination, for example.
Even except for the above-described MIM thin-film electron emitter, there are known, as thin-film electron emitters, a MIS (Metal-Insulator-Semiconductor) type (described in, for example, Journal of Vacuum Science and Technologies B, Vol. 11, pp. 429–432) using a semiconductor as a base electrode, one (described in, for example, Japanese Journal of Applied Physics, Vol. 36, Part 2, No. 7B, pp L939–L941 (1997)) using a semiconductor-insulator multi-layer film as a tunneling insulator, one (described in, for example, Japanese Journal of Applied Physics, Vol. 34, Part 2, No. 6A, pp. L705–L707 (1995)) using porous silicon as a tunneling insulator, etc.
A display apparatus using a thin-film electron-emitter matrix makes no use of a shadow mask like a cathode-ray tube (Cathode-ray tube; CRT) and has no beam deflection circuit. Therefore power consumption thereof is slightly lower than that of CRT or the same degree as that.
Power used up or consumed by the thin-film electron-emitter matrix is roughly calculated according to a conventional driving method for the display apparatus using the thin-film electron-emitter matrix.
FIG. 14 is a diagram showing a schematic configuration of a conventional thin-film electron-emitter matrix.
Thin-film electron-emitter elements 301 are respectively formed at points where row electrodes (base electrodes) 310 and column electrodes (top electrodes) 311 intersect respectively.
Incidentally, while the thin-film electron-emitter matrix is illustrated with 3 rows and 3 columns in FIG. 14, the thin-film electron-emitter elements 301 are actually placed by the number of pixels constituting a display apparatus, or the number of sub-pixels in the case of a color display apparatus.
Namely, as the number of rows N and the number of columns M, N ranges from several hundreds of rows to a few thousand rows and M ranges from several hundreds of columns to a few thousand columns as typical examples, respectively.
Incidentally, while one pixel is formed of a combination of respective sub-pixels of red, blue and green in the case of a color image display, ones equivalent to sub-pixels employed in the case of the color image display will be called “pixels” in the present specification. In the present specification, the pixels or sub-pixels are also called “dots”.
FIG. 15 is a timing chart for describing the conventional method of driving the display apparatus.
A row electrode driving circuit 41 applies a negative polarity pulse (scan pulse) having amplitude (Vrow) to one of the row electrodes 310 (a selected scan electrode). Simultaneously column electrode driving circuits 42 apply positive polarity pulses (data pulses) each having amplitude (Vcol) to some (their corresponding selected column electrodes) of the column electrodes 311.
Since a voltage enough to emit electrons is applied to each thin-film electron-emitter element 301 in which the two pulses overlap each other, the electrons are emitted therefrom. The electrons excite each of phosphors to emit light therefrom.
In the case of the thin-film electron-emitter element 301 free of the application of the positive polarity pulse having the amplitude (Vcol) thereto, a sufficient voltage is not applied thereto and hence no electron emission is produced.
The row electrodes 310 to be selected, i.e., the row electrodes 310 to which the scan pulse is applied, are successively selected and the data pulses applied to the column electrodes 311 in association with rows for the selected row electrodes are also changed.
When all the rows are scanned in this way during one field period, an image corresponding to an arbitrary image can be displayed.
During a given period in one field, pulses of reverse polarity (reverse pulses) are respectively applied to all the row electrodes.
Thus, the thin-film electron-emitter elements 301 can be operated stably.
Dissipation power of each driving circuit is calculated according to the conventional driving method when the electrostatic capacitance per one of the thin-film electron-emitter elements 301 is represented as Ce, the number of the column electrodes 311 is represented as M and the number of the row electrodes 310 is represented as N.
The dissipation power is equivalent to power used up or consumed to charge the electrostatic capacitance of each driven element and discharge the same therefrom. The dissipation power does not contribute to light emission.
Dissipation power produced with the application of scan pulses will first be determined.
Dissipation power at the time that a pulse having amplitude (Vrow), is applied to the corresponding row electrode 310 once, is expressed in the following equation (1):M·Ce·(Vrow)2  (1)
Assuming that the number of refreshing images (field frequency) per second is given as f, the whole dissipation power (Prow) for N row electrodes is expressed in the following equation (2):Prow=f·N·M·Ce·(Vrow)2  (2)
Similarly, dissipation power (Pr) consumed with the application of reverse pulses is given by the following equation (3):Pr=f·N·M·Ce·(Vr)2  (3)
where Vr indicates the voltage amplitude of the reverse pulse applied to the row electrode 310.
Since N thin-film electron-emitter elements 301 are connected to one column electrode 311, the whole dissipation power (Pcol) for M column electrodes is given by the following equation (4) where pulse voltages are applied to all of the M column electrodes 311:Pcol=f·M·N·(N·Ce·(Vcol)2)  (4)
Since the pulses are applied to the column electrodes N times during a screen-refreshing period (one field period), Pcol is additionally multiplied by N as compared with Prow.
Incidentally, when pulse voltages are respectively applied to m of the M column electrodes 311, M in the equation (4) is substituted with m.
Using f=60 Hz, N=480, M=1920, Ce=0.1 nF, and Vrow=Vr=Vcol=4V as typical values, for example, results in Prow=Pr=0.09 [W] and Pcol=42[W].
Since, in this case, the power consumption of the thin-film electron-emitter element per se becomes about 1.6[W], the total power consumption results in about 44[W]. This is practically problem-free power consumption.
It is however understood that when it is desired to further achieve low power consumption, a reduction in dissipation power Pcol consumed with the application of the data pulse is effective.
Thus, even the prior art presents no problem in terms of power consumption when used as the display apparatus in a similar use as the CRT.
However, the feature of the display apparatus using the thin-film electron emitters is to enable the implementation of a thin flat-panel display.
This type of thin flat-panel display has a use as for a portable display apparatus. In this case, power consumption may preferably be further reduced.